1. Field of the Invention
The adhesive resin systems of the present invention are in the field of curable thermoplastic-thermoset adhesives based on polyamide resins in combination with epoxy resins. The present invention is also in the field of adhesive resin formulations which are particularly tailored to meet specific application needs, which in the case at hand is for use in making packaging which is sterilizable.
2. Brief Description of the Prior Art
Dimer-based polyamides useful for curing epoxy resins are known, as described, e.g., in U.S. Pat. Nos. 3,062,773 (Rogier); 2,999,826 (Peerman et al.); 2,930,773 (Renfrew et al.); 2,890,184 (Foerster); 2,881,194 (Peerman et al.); and 2,707,708 (Wittcoff). However, the polyamides disclosed in these references are not solids at ambient temperatures, but are liquids having a high amine number. The liquid polyamides are mixed with liquid epoxies and allowed to cure. The products possess little cohesive strength until sufficient curing occurs. While certain of the cured products may be useful as adhesives, they suffer from the shortcomings of being usually rigid and having limited ductility.
It is also known to blend a thermoplastic polyamide with an epoxy resin to produce an adhesive, e.g., as disclosed in U.S. Pat. No. 2,867,592 (Morris et al.). However, the amount of epoxy resin added is limited to a proportion effective to link small numbers of polyamide resin molecules together without providing sufficient cross linking to cause curing. Thus, such a blend is not a thermoset and is not capable of being cured to an infusible final product.
U.S. Pat. No. 3,488,665 (MacGrandle et al.) teaches a process wherein polyamides are blended with epoxies to provide a product which cures after application to the substrate; however, the product is used to provide a hard, stiff coating for flexible sheet material. Curable adhesives have been made using polyamide resins and epoxy resins as in U.S. Pat. No. 2,705,223 (Renfrew et al.). But the Renfrew compositions possess inferior properties when applied as adhesives. For example, the Renfrew compositions do not possess good adhesive strength upon cure and provide limited working time after the mixing of the components. In addition, such compositions exhibit poor flexibility, and poor adhesive resistance to heat, water and organic solvents when applied to substrates at ambient temperature.
U.S. Pat. No. 4,082,708 (Mehta) teaches an adhesive system comprising an epoxy resin and a polyamide wherein the polyamide is derived substantially from primary and tertiary amines; specifically, the Mehta polyamides are derived from 1,4-bis-primary amino lower alkyl piperazines having terminal primary amine groups. Although it is suggested that secondary amines can be utilized in making the polyamides as chain extenders and flexibilizers, it is taught that the secondary amines are less desirable reactants and should be buried within the polyamide structure.
European Patent Application Publication No. EP 0 157 478 describes adhesive compositions prepared by mixing solid particles of polyamide resins with solid particles of epoxy resins. If desired, the composition also contains a polymeric binder. Prior to application as an adhesive, the polyamide and epoxy composition is preferably dispersed in a non-solvent.
Hastings et al., U.S. Pat. No. 3,945,964 describes stable aqueous emulsions of epoxy resins containing, as emulsifying agents, ethylene oxide/propylene oxide block polymers of specified formulae. Curing agents described as useful with the epoxy resin emulsion include polyamides.
Aelony et al., U.S. Pat. No. 2,899,397 describes mixed aqueous emulsions of polyamide and epoxy resins prepared by mixing a solution of epoxy resinous material with a solution of polymeric polyamides, combining the two solutions in the presence of an aqueous medium containing a water soluble organic carboxylic acid and agitating the combined solutions to produce an emulsion.
Wittcoff and Renfrew U.S. Pat. No. 2,811,495 discloses mixtures of suspensoids of polyamide resins and complex epoxide resins, but there is no suggestion of the compositions of the present invention.
Frihart U.S. Pat. No. 5,296,557 discloses two-component curable hot melt adhesive compositions in which the polyamide component is substantially free of piperazine-containing polyamines and comprises at least one branched diamine, with preferably an excess of free amine to acid groups. The epoxy component is preferably a multifunctional epoxy novolac resin. The thermoset final product has heat resistivity and may be applied to substrates such as nonwoven fabrics and polycarbonates.
Smith and Hagedorn U.S. Pat. No. 5,095,058 discloses polyamide resin dispersions for hot melt adhesive applications, which have a low acid number achieved with a neutralizing base such as KOH, a nonionic or other surfactant to promote emulsification, and a stabilizing agent such as glycine.
Smith and Frihart WO 92/19677, published Nov. 12, 1992, contains much the same disclosure as the U.S. Pat. No. 5,095,058, but additionally includes an epoxy component, e.g., the composition of Example 2 contains UNI-REZ 2636 and EPI-REZ W55-5003. There is also disclosed the addition of a neutralizing acid or base which is present in an amount effective to neutralize residual acid or base in the polyamide resin.
Polyamide resin microdispersions and methods for the manufacture thereof are also disclosed in WO 92/16579, published Oct. 1, 1992. The polyamide microdispersions disclosed therein have improved stability against phase separation and gelation. Additionally, they are capable of forming clear, continuous films at ambient or near ambient temperatures.
One problem with many of the prior art epoxy/polyamide resin compositions is the rapid reactivity of the mixed resins, producing in a relatively short amount of time an infusible, insoluble composition. Where such compositions are intended for use as adhesives in the type of application to which the method of the present invention is directed, this brief reaction time requires that the resin mixture be prepared at point of use and in relatively small quantities. Attempts have been made to lengthen the shelf life of these compositions; however, such attempts have met with little success. In addition, relatively large amounts of volatile organic solvents used in these processes (e.g., toluene, isopropanol, methylethyl ketone) are flammable and environmentally undesirable.
The present invention provides an improvement over the prior art in that it provides as a single, uniform composition, a curable polyamide and epoxy resin mixture, i.e., an adhesive resin system with good stability properties, and with the ability to be cured, almost immediately after the application of heat, to a final product that is resistant to high heat and humidity conditions, has good initial strength properties, improved adhesive and cohesive strength, and resistance to remelting (or softening with heat), solvent attack and moisture damage. These properties are obtained almost immediately upon heat sealing of the adhesive resin system to a substrate with readily usable and available equipment and conditions, whereby curing to the final adhesive product and establishment of the adhesive bond is obtained. Furthermore, the adhesive resin system, prior to curing, possesses excellent shelf life, with stability to premature curing in excess of one year. These characteristics are especially valuable in cured adhesive products which must perform under challenging conditions of temperature, humidity, and mechanical stress. In addition, the adhesives of the present invention possesses greater ductility and flexibility and are easier to employ. The adhesives are especially useful with difficult to bond substrates, including polycarbonate resins.
There is an ongoing need for new and/or better curable epoxy and polyamide resin compositions, particularly compositions having all of the properties heretofore required of an adhesive, but additionally being able to bond to difficult surfaces such as polycarbonate, being curable at elevated temperature, but once cured, also being sufficiently heat and moisture resistant to successfully withstand sterilization conditions, within a short time after heat sealing with readily usable and available equipment and conditions. The present invention is directed to these important ends.